1. FIELD OF THE INVENTION
The present invention relates to a process for producing a superconductor of an oxide system.
2. DISCUSSION OF BACKGROUND
Conventional superconductors are most commonly of a metallic type. Among them, Nb.sub.3 Ge had the highest transition temperature (critical temperature) for superconductivity at a level of 23.2 K.
On the other hand, with superconductors of a metal oxide system, the critical temperature was usually lower than the superconductor of a metallic system and was at a level of 13 K even with BaPb.sub.1-x Bi.sub.x O.sub.3 which had the highest critical temperature.
Recently, however, as a superconductor of an oxide system having a high critical temperature, a material of a La--Sr--Cu--O system (critical temperature: about 40 K) and a material of a Y--Ba--Cu--O system (critical temperature: about 90 K) have been discovered and have created a boom for the development of materials having high temperature superconductivity.
For the preparation of these superconductors of an oxide system, a so-called dry (powder) method and a coprecipitation method have been commonly and widely used as disclosed in Zeitschrift fur Physik B-Condensed Matter, Vol. 64, p.189 (1986) and Japanese Journal of Applied Physics, Vol. 26, No. 3, PL 196 (1987) and ditto, Vol. 26, No. 4, PL 314 (1987).
The dry method is a method wherein powder materials of oxides or carbonates of e.g. La, Y, Ba, Sr and Cu are mechanically mixed by means of a mortar or a mill, followed by sintering to obtain a sintered product of oxides.
The coprecipitation method is a method wherein nitrates of the above-mentioned metals were uniformly mixed and dissolved in an aqueous medium, and then oxalic acid or ammonia is added to simultaneously form the respective precipitates in the form of a mixture.
The conventional dry method as mentioned above has drawbacks such that even when special grade reagents are used as the respective powders, their purity is not so high at a level of from 98 to 99.9% by weight, and impurities are included in the superconductor after sintering, and there is a limitation in the uniformity of the mixed state attainable by mere mechanical mixing of the respective powders and it is impossible to uniformly mix them in a strict sense, whereby unwanted phases other than the high temperature superconducting phase are likely to be present in the superconductor after sintering. Accordingly, the superconductor of an oxide system prepared by such a dry method, is obliged to have poor superconducting characteristics such that the critical temperature is low, the transition temperature range is wide, and the critical current density is small. Further, the sintering temperature is required to be high, and it takes a long time for the sintering.
In the coprecipitation method, since alkaline earth metal ions hardly precipitate unless the aqueous solution of the mixture is made alkaline, ammonia or the like is added to facilitate the precipitation of alkaline earth metal ions. However, it has a drawback that if ammonia or the like is added, copper forms complex ions, which can hardly be precipitated. Therefore, it has been pointed out that the coprecipitation method is not suitable to obtain a superconductor of an oxide system having a specific desired composition (Applied Physics, Vol. 56, No. 5, p. 606 (1987)). Thus, the coprecipitation method also has a problem in obtaining a sintered product having good superconducting characteristics.
Recently, a new high temperature oxide superconductor (constituting elements Bi--Sr--Ca--Cu--O) containing no rare earth elements has been reported at a press conference on Jan. 21, 1988 and published on Jan. 22, 1988 by Kinzoku Zairyo Gijutsu Kenkyusho, and has created a further drive for the research of new superconducting materials.
This superconductor has a superconducting phase with a critical temperature higher than the above-mentioned superconductor of a Y--Ba--Cu--O system discovered by professor Chu of Houston University and contains no rare earth elements, and it shows superconducting characteristics even when dipped in water and is stable and readily reproducible. Further, it does not contain Ba as opposed to the oxide superconductor of a YBCO system and is free from the possibility that Ba turns into BaCO.sub.3 during the sintering, and it is therefore possible to set the sintering temperature at a low level. Thus, it is considered to be a practical superconductor. However, this superconductor of a Bi--Sr--Ca--Cu--O system is also produced by a dry system having the above-mentioned problem.
Further, a new superconductor of an oxide system containing thallium has been discovered by professor Hermann of Arkansas University in the United States, which has further promoted the research for new superconducting materials. This superconductor of a Tl--Ca--Ba--Cu--O system has a critical temperature higher than the superconductor of a Y--Ba--Cu--O system and can be regarded as a more practical superconducting material.
However, this superconductor was also prepared by a dry method wherein powder materials of oxides or carbonates of thallium, calcium, barium and copper were mixed by means of a mortar or a mill, followed by sintering to obtain a sintered product of oxides.